1. Field of the Invention
The present invention relates to a mobile terminal apparatus used in an area in which services are provided by a plurality of cellular mobile communication systems different in communication standards, e.g., a digital cell phone system, analog cell phone system, and PCS (Personal Communication System), and to a system selecting method used to select an optimum system while this mobile terminal apparatus is in a standby state.
2. Description of the Related Art
Generally, when a mobile terminal apparatus used in a cellular mobile communication system is powered on, the apparatus searches for a control signal transmitted by a base station of a system desired by the user, on the basis of a list registered beforehand. The mobile terminal apparatus then performs processing for establishing synchronization with the base station transmitting the control signal acquired by the search. When this synchronization is established, the mobile terminal apparatus proceeds to a standby state. Once the apparatus proceeds to this standby state, no effective messages from the base station can be received over a predetermined period after that. The mobile terminal apparatus holds this synchronization established state with respect to the desired system until a “system lost” condition occurs or speech or data communication once started is terminated.
With this configuration, however, if the reception quality deteriorates in the standby state, this deteriorated reception quality state continues until the “system lost” condition occurs or communication once started is terminated. In this case, if, among peripheral base stations which belong to the same system as the base station which has established the synchronization, there is a peripheral base station having higher reception quality than that of the former base station, idle handoff is performed for the latter base station. This can improve the reception quality. However, if there is no peripheral base station which belongs to the same system as the base station which has established the synchronization, or if no high-quality signals can be received from a peripheral base station which belongs to the same system, no idle handoff can be performed. Accordingly, the mobile communication terminal apparatus in a situation like this keeps standing by with the deteriorated reception quality. This sometimes interferes with connection control for an outgoing call or incoming call.